Save and expose to the API fee information about transactions when available

[NicolasDorier]

This PR is saving in the txs.metadata some useful information such as fee, virtual size and fee rate.

   "metadata": {
    "vsize": 141,
    "fees": 2820,
    "feeRate": 20.0
  }

/v1/cryptos/{cryptoCode}/transactions/{txId} will now also look into the node's getrawtransaction if the data isn't found in the database, regardless of index availability.

I also added getmempoolentry to the list of whitelisted RPC methods exposed by NBXplorer.

It also improve a bit performance by not querying database twice when checking for a match, if it is unsuccessful.